This invention relates to the preparation of diaryl carbonates by oxidative carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply xe2x80x9ccarbonylationxe2x80x9d for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a Group VIIIB metal, i.e., a metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
A further development in the carbonylation reaction, including the use of compounds of other metals such as lead or cerium as cocatalysts, is disclosed in various patents including U.S. Pat. No. 5,498,789. Also required according to that patent is the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package.
The commercial viability of the carbonylation reaction would be greatly increased if a less expensive compound could be substituted for the quaternary ammonium or phosphonium halide. It has been discovered, however, that substitution of such compounds as sodium bromide normally results in the isolation of the desired diaryl carbonate in low or insignificant yield.
In U.S. Pat. Nos. 5,543,547 and 5,726,340, the use of carbonylation catalyst systems including palladium or an analogous metal, various cocatalytic metals which may include cerium, lead or cobalt, and an alkali metal or quaternary ammonium bromide is disclosed. Also present may be materials characterized as inert solvents. These may be aliphatic or alicyclic hydrocarbons such as hexane, heptane or cyclohexane; chlorinated aliphatic hydrocarbons such as methylene chloride or chloroform; aromatic hydrocarbons such as toluene or xylene; chlorinated aromatic hydrocarbons such as chlorobenzene; ethers such as diethyl ether, diphenyl ether, tetrahydrofuran or dioxane; esters such as ethyl acetate or methyl formate; nitroaromatic compounds such as nitrobenzene; or acetonitrile. There is no suggestion, however, that yields of diaryl carbonate are in any way improved by the use of any of these xe2x80x9csolventsxe2x80x9d in a halide-containing catalyst package.
U.S. Pat. No. 5,380,907 discloses the use of a nitrile in combination with palladium and a manganese or copper cocatalyst. The result is an increase in yield, but yields are still too low to permit contemplation of commercial use for the disclosed catalyst systems.
It is of interest, therefore, to develop catalyst systems which include an inexpensive halide compound and which can efficiently produce diaryl carbonates.
The present invention provides a method for preparing diaryl carbonates which includes a relatively inexpensive halide and a promoter compound which maximizes the effectiveness of said halide. Also provided is a catalyst composition useful in such a method.
In one of its aspects, the invention is a method for preparing a diaryl carbonate which comprises contacting at least one hydroxyaromatic compound with oxygen and carbon monoxide in the presence of an amount effective for carbonylation of at least one catalytic material comprising:
(A) a Group VIIIB metal or a compound thereof,
(B) at least one alkali metal halide, and
(C) an amount effective to optimize diaryl carbonate formation of a promoter compound which is at least one C2-8 aliphatic or C7-10 aromatic mono- or dinitrile.
Another aspect of the invention is catalyst compositions comprising components A, B and C as described above, and any reaction products thereof.
Any hydroxyaromatic compound may be employed in the present invention. Monohydroxyaromatic compounds, such as phenol, the cresols, the xylenols and p-cumylphenol, are generally preferred with phenol being most preferred. The invention may, however, also be employed with dihydroxyaromatic compounds such as resorcinol, hydroquinone and 2,2-bis(4-hydroxyphenyl)propane or xe2x80x9cbisphenol Axe2x80x9d, whereupon the products are polycarbonate oligomers.
Other essential reagents in the method of this invention are oxygen and carbon monoxide, which react with the phenol to form the desired diaryl carbonate. They may be employed in high purity form or diluted with another gas such as nitrogen, argon, carbon dioxide or hydrogen which has no negative effect on the reaction.
For the sake of brevity, the constituents of the catalyst system are defined as xe2x80x9ccomponentsxe2x80x9d irrespective of whether a reaction between said constituents occurs before or during the carbonylation reaction. Thus, the catalyst system may include said components and any reaction products thereof.
Component A of the catalyst system is one of the Group VIIIB metals, preferably palladium, or a compound thereof. Thus, useful palladium materials include elemental palladium-containing entities such as palladium black, palladium/carbon, palladium/alumina and palladium/silica; palladium compounds such as palladium chloride, palladium bromide, palladium iodide, palladium nitrate, palladium acetate and palladium 2,4-pentanedionate; and palladium-containing complexes involving such compounds as carbon monoxide, nitrites and olefins. Preferred in many instances are palladium(II) salts of organic acids, most often C2-6 aliphatic carboxylic acids, and palladium(II) salts of xcex2-diketones. Palladium(II) acetate and palladium(II) 2,4-pentanedionate are generally most preferred. Mixtures of the aforementioned palladium materials are also contemplated.
Component B is at least one alkali metal halide. The alkali metal bromides such as lithium bromide, sodium bromide and potassium bromide are preferred, with sodium bromide often being most preferred by reason of its particular suitability and relatively low cost.
Component C is a promoter compound, said compound being at least one C2-8 aliphatic or C7-10 aromatic mono- or dinitrile. Illustrative mononitriles are acetonitrile, propionitrile and benzonitrile; illustrative dinitriles are succinonitrile, adiponitrile and benzodinitrile. Mononitriles are generally preferred, with acetonitrile and adiponitrile being most preferred.
It should be noted that contrary to the suggestion of some of the prior art identified hereinabove, the function of component C in the present invention is not simply that of a solvent. Rather, the nitrite is an active catalyst constituent which improves the yield of the desired diaryl carbonate.
In a highly preferred embodiment of the invention, there is also present in the catalyst system (D) at least one cocatalyst which is a compound of a non-Group VIIIB metal, preferably one which is soluble in the liquid phase under the reaction conditions. Numerous other metal compounds are known in the art to be active as carbonylation cocatalysts, and any compound having such activity may be used according to the present invention provided an improvement in diphenyl carbonate production, usually yield, is achieved thereby.
Illustrative cocatalytic metals include cerium, titanium, cobalt, copper, zinc, manganese, iron and lead, which may be used singly or in combination. For the purposes of this invention the preferred cocatalysts are those containing metals other than Group VIII metals; that is other than iron, cobalt and nickel. More preferred are compounds of lead, particularly when used alone or in combination with titanium or cerium compounds. It should be noted, however, that component C is not effective to optimize diaryl carbonate formation for all possible permutations of component D; the combined effectiveness of the two for this purpose may be determined by simple experimentation.
Examples of lead compounds which may be employed are lead oxides such as PbO and Pb3O4xe2x88x92; inorganic lead salts such as lead(II) nitrate; lead carboxylates such as lead(II) acetate, lead(II)propionate and lead(IV) acetate; lead alkoxides and aryloxides such as lead(II) methoxide and lead(II) phenoxide; and lead salts of xcex2-diketones such as lead(II) 2,4-pentanedionate. Mixtures of the aforementioned lead compounds may also be employed. The preferred lead compounds are lead(II) oxide, lead(II) aryloxides and lead(II) 2,4-pentanedionate. The preferred compounds of other metals are, for the most part, salts of xcex2-diketones and especially 2,4-pentanedionates.
In addition to the aforementioned reactants and catalyst system, it is strongly preferred for a desiccant to be present in the reaction system. The preferred desiccants are non-reactive materials such as molecular sieves, as illustrated by 3-xc3x85ngstrom (hereinafter xe2x80x9c3Axe2x80x9d) molecular sieves. They are usually isolated from the other reactants, as by presence in a basket mounted to a stirrer shaft or the like.
Component A is most often present in the amount of about 0.1-100,000 ppm, preferably 1-1,000 ppm, of Group VIIIB metal based on hydroxyaromatic compound, and component B in the amount of about 1-2,000 gram-atom of total metal per gram-atom of palladium in component A. Component D, when employed, is generally present in the amount of about 1-100 gram-atoms of total metal per gram-atom of palladium in component A.
The role of component C in the composition and method of the invention is believed to be to increase the degree of dissociation and ionization of the halide anion of component B, perhaps by forming a complex with the cationic portion of said component, although the invention is in no way dependent on this or any other theory of operation. The amount of component C employed will be an amount effective to optimize diaryl carbonate formation, in general by increasing the yield of the desired diaryl carbonate as evidenced, for example, by an increase in xe2x80x9cturnover numberxe2x80x9d; i.e., the number of moles of diaryl carbonate formed per gram-atom of palladium present. This amount is most often about 1 prat by weight of component C per 1-15, preferably about 1-6, parts of hydroxyaromatic compound.
The method of the invention is preferably conducted in a reactor in which the hydroxyaromatic compound and catalyst system are charged under pressure of carbon monoxide and oxygen and heated. The reaction pressure is most often within the range of about 1-500 and preferably about 1-150 atm. Gas is usually supplied in proportions of about 2-50 mole percent oxygen with the balance being carbon monoxide, and in any event, the mole percentage of oxygen used should be outside the explosion range for safety reasons. The gases may be introduced separately or as a mixture. Reaction temperatures in the range of about 60-150xc2x0 C. are typical. In order for the reaction to be as rapid as possible, it is preferred to substantially maintain the total gas pressure and partial pressure of carbon monoxide and oxygen, as described, for example, in U.S. Pat. No. 99,734, until conversion of the hydroxyaromatic compound is complete.
The diaryl carbonates produced by the method of the invention may be isolated by conventional techniques. It is often preferred to form and thermally crack an adduct of the diaryl carbonate with the hydroxyaromatic compound, as described in U.S. Pat. Nos. 239,106 and 5,312,955.